1. Field of the Invention
The present invention relates to a piezoelectric resonator, a piezoelectric filter including a piezoelectric resonator, a duplexer, a communication apparatus, and a method for manufacturing a piezoelectric resonator.
2. Description of the Related Art
A piezoelectric resonator provided with a piezoelectric thin film on a substrate has been used as a filter for a communication device.
Some piezoelectric resonators include piezoelectric vibration portions in which a piezoelectric thin film is sandwiched between opposed upper and lower electrodes, and a high-frequency signal is applied to the sandwiched piezoelectric thin film via the electrodes such that the piezoelectric thin film vibrates in a thickness vibration mode. Examples of such piezoelectric resonators include a diaphragm type piezoelectric resonator having an opening in a substrate which is covered with a diaphragm made of laminated thin films, and the piezoelectric vibration portion is supported by this diaphragm.
FIG. 42 is a sectional view showing the structure of a conventional piezoelectric resonator (see, for example, Japanese Unexamined Patent Application Publication No. 2001-279438).
This piezoelectric resonator includes a substrate 201, a plurality of substrate films 202 and 203, a pair of electrodes 204 and 206, and a piezoelectric thin film 205. Since the substrate film is defined by a plurality of layers as described above, each substrate film shares functions, for example, chemical stability and adhesiveness, and as a result, improvement of the performance and reliability of the piezoelectric resonator are achieved.
Furthermore, some piezoelectric resonators include piezoelectric vibration portions in which a piezoelectric thin film is sandwiched between opposed upper and lower electrodes, and a high-frequency signal is applied to the sandwiched piezoelectric thin film via the electrodes and, therefore, the piezoelectric thin film vibrates in a thickness vibration mode. Examples of such piezoelectric resonators include a diaphragm type piezoelectric resonator having a structure in which an opening of a substrate is covered with a diaphragm made of laminated thin films, and the piezoelectric vibration portion is supported by this diaphragm.
In some piezoelectric resonators having a diaphragm structure, the residual stress during diaphragm formation is adjusted to be nearly zero and, thereby, the resonant characteristics are improved, or the diaphragm surface is flattened with high precision and, thereby, the resonant characteristics are improved (see, for example, Japanese Unexamined Patent Application Publication No. 2000-244030).
Japanese Unexamined Patent Application Publication No. 2001-279438 discloses an example of a piezoelectric resonator in which the lower substrate film 202 of a plurality of substrate films is made of SiNx, the upper substrate film is made of Al2O3, and the piezoelectric thin film is made of ZnO.
However, regarding the configuration of such a conventional piezoelectric resonator, since each of the temperature coefficient of resonant frequencies (TCF) of SiNx, Al2O3, and ZnO has a negative value, when the substrate films 202 and 203 made of SiNx and Al2O3 are combined with the piezoelectric thin film 205, the temperature coefficient of resonant frequency in the basic mode is deteriorated.
Furthermore, regarding the piezoelectric resonator in which the residual stress during diaphragm formation is adjusted to be nearly zero, when the diaphragm surface is not flat, the crystallinity and orientation property of the piezoelectric vibration portion are reduced and, therefore, the resonant characteristics cannot be sufficiently improved. Regarding the piezoelectric resonator in which the diaphragm surface is flattened, when the stress remains, the resonant characteristics cannot be sufficiently improved.